The invention relates to the use of salt-based compounds as additives in electrolytes for improving the properties of electrochemical cells.
Lithium ion batteries are amongst the most promising systems for mobile applications. The areas of application extend from high-quality electronic equipment (for example mobile telephones, camcorders) to batteries for electrically driven vehicles.
These batteries consist of a cathode, an anode, a separator and a non-aqueous electrolyte. The cathode is typically Li (MnMez)2O4, Li(CoMez)O2, Li (CoNixMez)O2 or other lithium intercalation and insertion compounds. Anodes can consist of-lithium metal, carbon, graphite, graphitic carbon or other lithium intercalation and insertion compounds or alloy compounds. The electrolyte can be a solution containing lithium salts, such as LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2 or LiC(CF3SO2)3 and mixtures thereof, in aprotic solvents.
A multiplicity of additives for use in lithium ion batteries is mentioned in the literature. For example, in EP 0759641 and U.S. Pat. No. 5,776,627, organic aromatic compounds, such as bi-phenyl, substituted thiophenes and furans, and in EP 0746050 and EP 0851524, substituted anisole, mesitylene and xylene derivatives are added to the electrolyte in order to increase the safety of the battery in the case of, overcharging. For the same purpose, U.S. Pat. No. 5,753,389 uses organic carbonates as additives In order to improve the cycle stability, organic boroxines are added in EP 0856901. However, all these additives have some crucial disadvantages. Organic substances, as used in the specifications mentioned here, generally have low flash points and low explosion limits.
AdditiveExplosion limit [%]Flash point [° C.]Thiophene1.5-12 −9Anisole0.34-6.3 43Mesitylene1-654Furan 2.3-14.3−35
Since the use of electrochemical cells and in particular the occurrence of faults (for example short-circuiting, mechanical damage) is always accompanied by warming, escape of the electrolyte represents an additional source of danger.